The invention relates to a method for the production of a composite multilayer material for sliding elements, in which a bearing metal layer and an intermediate layer are applied to a backing layer and an overlay is electrodeposited on the intermediate layer. The invention also relates to a corresponding composite multilayer material for sliding elements.
Known composite multilayer materials consist of a stable backing layer, generally of steel, with a cast-on, sintered-on or rolled-on bearing metal layer, which is generally copper- or aluminium-based, and an overlay deposited thereon, e.g. a lead-tin-copper alloy. In general, a diffusion barrier layer, e.g. of nickel, is arranged between the overlay and the bearing metal. Such composite multilayer materials are known from DE-PS 830 269, for example.
The overlay assumes a plurality of functions. It may render hard, abrasive particles harmless by embedding and it serves in conformation to the shaft during the running-in phase. It provides a degree of corrosion protection for the bearing metal and exhibits emergency running properties in the case of an oil shortage.
The life cycle of the overlay comprises the following phases:
running-in phase with a relatively high level of wear
continuous running phase with constant relatively low rate of wear
phase after layer has been totally worn away, with increased scuffing sensitivity.
The hardness of conventional overlays represents a compromise between embeddability and emergency running behaviour, i.e. low layer hardness, and wear resistance, i.e. high layer hardness.
To optimise overlay performance, for example, a special structure has been developed which is built up from alternate layers of a softer material and a harder material. Such an overlay is known from DE 39 36 498 A1, for example. An electric current is established in the electroplating bath with a density of 0-80 A/dm2 and a potential of xe2x88x921.5 to +0.5 volts. Alternate layers of CuSnPb (soft layer) and Cu or CuSn (hard layer) are deposited from a lead-tin-copper bath. Deposition is achieved from a single fluoroborate-containing bath by changing the deposition parameters.
This layer arrangement, which may comprise up to several hundred layers, the individual layers each having a thickness of only a few xcexcm, has the serious disadvantage that copper and tin interdiffuse, creating brittle intermetallic phases which exhibit unfavourable tribological properties and additionally have a tendency towards brittle fracture.
A homogenisation-based method is known from DE 41 03 117 C2 for producing sliding elements having an overlay of a ternary or binary white metal alloy. This method has the disadvantage, however, that the concentration of soft component or hard component cannot be varied as desired over the layer thickness, since the law of diffusion ensures that a concentration distribution is always established in accordance with an e-function. In the case in particular of a steeply falling e-function, absolutely no more variation in hardness is obtained in the lower overlay region. A further disadvantage is that an additional method step is necessary to adjust the concentration gradient after electrodeposition of the overlay.
The object of the invention is to provide a method with which it is possible to produce a composite multilayer material which exhibits optimum properties during its entire service life.
The invention is based on the finding that optimum properties are obtained in particular when the overlay exhibits optimum hardness throughout the service life of the sliding element made from the composite multilayer material. This is achieved in that, in the case of the composite multilayer material according to the invention, the overlay exhibits a hardness which increases continuously from its surface in the direction of the bearing metal layer.
The overlay, and thus the composite multilayer material according to the invention, exhibit a layer hardness conformed to the respective operating phase: low hardness during the running-in phase, increasing hardness during the continuous running phase, such that the service life may thus be considerably increased overall. It has proved possible to improve service life by a factor of 1.5 to 2 in comparison with conventional overlays.
Such an overlay requires a method by means of which hardness may be specifically adjusted over the layer thickness.
The object is achieved according to the method in that a lead-free alloy with at least one hard and one soft component is deposited as overlay, the current density being modified within the range of from 0.3-20 A/dm2 during the deposition process and/or the temperature of the electroplating bath being modified within the range of from 15xc2x0 C. to 80xc2x0 C.
The method according to the invention permits of several alternatives.
According to a first variant, the method is performed at a constant temperature and the current density is modified, preferably increased, during the deposition process. The result of this is that, as the current density increases, the soft component is preferentially deposited, which leads to a larger proportion of soft component in the overlay.
According to a second variant of the method, the current density is set at a certain value and the temperature is modified. It has been shown that there is a correlation between temperature and deposition behaviour of the hard and soft components to the effect that, as the temperature increases, deposition of the hard component is favoured. In order to establish the desired hardness gradient, it is necessary, according to a first embodiment of this second method variant, to bring the temperature down from a high value during deposition. This means that the electroplating bath has to be cooled.
Since, from the point of view of plant, it is simpler to heat up the electroplating bath, a second embodiment of this second method variant involves the preferred addition of a polariser to the electroplating bath. It has emerged that an unsaturated carboxylic acid-based additive is suitable for this purpose. The polariser preferably contains approx. 30% carboxylic acid and up to a third aryl polyglycol ether and/or alkyl polyglycol ether, the rest consisting of water. This additive is preferably added in amounts of up to 10% relative to the total amount of electroplating bath.
This xe2x80x9cpolariserxe2x80x9d additive effects a modification of the potential of the harder component, with the result that deposition of the harder component is reduced as the temperature increases.
The method variants may also be combined with one another, both the current density and the temperature then being modified during the deposition process.
The method has the advantage of making it possible to produce the layer with the respective running properties by electrodeposition from a single bath. Use of a plurality of electroplating baths, e.g. of different temperatures, is not ruled out.
Current density and/or temperature may be modified in stages, such that a laminar structure is established within the electrodeposited overlay. However, it has been shown that sudden changes in hardness are not always advantageous from the point of view of running properties. A continuous variation in hardness, i.e. a hardness gradient, is therefore preferred. Accordingly, the current density and/or temperature Is preferably continuously modified.
The current density is preferably increased at a rate of from 0.1 to 0.5 A/(dm2 min).
The temperature is preferably modified at a rate of from 1xc2x0 to 5xc2x0 C./min.
The current density range covered during deposition depends on the alloy used. If a binary alloy of tin and copper is deposited, the current density preferably varies within the range of from 0.5-10 A/dm2. Suitable binary alloys include in particular CuAg, AgCu, SnCu, CuSn, SnBi or SnAg.
Deposition is preferably performed using a fluoroborate-free electroplating bath.
The method according to the invention makes it possible to adjust the hardness of the overlay upwards in the range of from 10 HV to 150 HV.
The bath composition is such that alloys of high and low hardness may be deposited.
The proportion of hard component preferably increases from the overlay surface in the direction of the bearing metal layer from 1 wt. % to 20 wt. %. The overlay alloy may additionally contain 0.1 wt. % to 5 wt. % nickel and/or cobalt. This addition has the effect of stabilising diffusion in the binary systems.
An intermediate layer serves as diffusion barrier and coupling agent and to improve wear and fatigue resistance. It preferably consists of nickel, SnNi, Ni+SnNi (two layers), Co or Fe. The tin content of the SnNi alloy layer preferably amounts to 65 to 75%.
Deposition of the intermediate layer may likewise be performed using electroplating or electroless (autocatalytic) plating. The bearing metal layer may be sintered or cast.